Stabilizing synthetic rubbers



I United States 2,834,752 Patented May 13, 1958 nice STABILIZINGSYNTHETIC RUBBERS Arnold R. Davis, Riverside, and FrankA. V. Sullivan,

Darien, Conn., assignors to American Cyanamid Company, New York, N. Y.,a corporation of Maine No Drawing. Application January 27, 1954 SerialNo. 406,596

4 Claims. (Cl. 260-4535) linkage and gel formation during drying andstorage. As

such it is a continuation-in-part of application for United StatesLetters Patent Serial No. 106,737, filed July 25,

1949, of Arnold R. Davis and Frank A. V. Sullivan, the applicantsherein, now abandoned.

- One of the difiiculties encountered in the manfacture of syntheticrubber-like polymers and copolymers is their lack of stability to gelformation caused by further polymerization, cyclization, cross-linkageand the like. After polymerization and coagulation, by whatever methodor combination of methods may be used, the product usually must bedried. Ordinarily, too, it is stored for a considerable period beforeits final utilization. During the drying and storage periods, it isoften found that crosslinkages develop and gel forms in the rubber.Also, cross-linkages and gel often develop during subsequent mechanicalprocessing, i. e., mastication, and the like. These phenomena areobjectionable in that they alter the properties of the product.

In the past, attempts have been made to prevent such changes in theproduct during drying, storage, mastication, etc. by the addition to thelatex, prior to compounding into a vulcanizable rubber composition, anyof various materials which exert a stabilizing effect. Unfortunately, ofthe materials available, none is sufliciently eifective in accomplishingthe desired result without at the same time discoloring the product. Forexample, various substituted amines and other chemicals have beenutilized. One of the most commonly used, and probably one of the best,is phenyl-beta-naphthyl amine. However, while effective as a stabilizer,it suifersfrom the objection that in admixture with the more commontypes of synthetic rubbers, it can only be used to produce a black ordarkcolored product, since it discolors light-colored or white rubberproducts very badly when exposed to light.

There remains, then, a commercial demand for materials which effectivelystabilize synthetic rubbers against cross-linkage and gel formationduring drying, storage and processing. Particularly is there a demandfor effective stabilizing agents suitable for use in light-colored orwhite rubber products which will not discolor the final product. It is,therefore, the principal object of the present invention to devise aproduct for, and a method of, preventing further polymerization,cyclization, crosslinkage and the like, and gel formation in syntheticrub-' bers of the polymer and copolymer types. It is a further object toprovide a color stable, unvulcanized, sulfur vulcanizable syntheticrubber composition stabilized against gel formation.

Surprisingly enough, the objects of the present invention have beensuccessfully met by the use of'a novel group of hindered phenols. Theresult is particularly 2- astonishing in view of the fact that whilephenolic reactants previously have been tried for the purpose, they havenot always been found to be quite adequate.

In general, a compound as used for the purpose of the present inventionmay be characterized 'as a 2,2-methylene-bis-(4,6-dialkylphenol) capableof being represented a by the formula R: 0H OH R:

in which R, is an alkyl radical of 1-3 carbon atoms and R is a tertiaryalkyl radical of from 4 to about 8 carbon atoms, the linkage from thetertiary carbon being directly to the phenol ring.

It is a particular advantage of the compounds of the present inventionthat their beneficial stabilizing or gel inhibiting effect extends oninto the manufacture of .plasticization of unvulcanized polymers.

finished products as well as providing exceptional stability duringdrying and storage prior to compounding. Acting ized rubber composition,is quite surprising. This is so inasmuch as it is recognized in the artthat there is no necessary correlation between the cross-linkage and thelike reaction occurring in unvulcanized synthetic rubbers and theoxidation reaction, whether occurring in the unvulcanized state or in avulcanized rubber composition. Sulfur vulcanizable synthetic rubberssuch as the butadiene polymers, butadiene-styrene copolymers, etc. areunsaturated and subject to oxidation in the unvulcanized state somewhatlike unvulcanized natural rubber. Synthetic rubbers, however, are alsosubject to changes in structure by further polymerization, cyclization,crosslinking and the like which result in formation of an in-' solublefraction (gel).

The polymerization, cyclization or cross-linkage reaction is known totake place in the absence of oxygen particularly under the influence ofheat (Rubber Chemistry and Technology, vol. 25, No. 1, pp. 33-35). A

trace of oxygen may catalyze the cross linking reaction' but largeamounts reduce the net rate (Ind. and Eng. Chemistry, August 1945, pp.770-775). This reaction is opposed to the oxidation reaction which canlead to polymerization, cyclization or cross linking reaction may gofaster than any oxidation reaction so that under some conditions a hardinsoluble and unusable unvulcanized polymer may result (India RubberWorld, vol. 108, 1943, pp. 45-51).

Just as it is recognized that there is no necessary correlation betweenthe two types of reactions, it is also recognized that there is nonecessary correlation between the utility of any particular material asa gel inhibitor in unvulcanized synthetic rubbers during drying, storageand use and as an antioxidant for preventing ageing of rubber invulcanize rubber compositions. Various materials have been found whichstabilize synthetic rubbet in varying degrees during manufacture andstorage thereof prior to compounding into a vulcanizable rubbercomposition. Some of these have also been found to In fact the functionas antioxidants in vulcanized rubber compositions while others areineffective in this capacity.

It is extremely diflicult, therefore, to attempt to predict with anydegree of success whether (1) an eifective stabilizer will or Will notbe an effective antioxidant, and vice versa; and (2) whether anineffective stabilizer will or will not be an ineifcctive antioxidant,and vice versa.

Treatment of vulcanizable and vulcanized rubber compositions with aspecified amount of a material as set forth above for the purpose ofreducing the normal rate of ageing, through oxidation, of vulcanizedrubber compositions forms the subject matter of United States Patent No.2,538,355 to Davis and Sullivan, the applicants herein.

It is a further advantage of the present invention that the stabilizersthereof may be simply and economcially prepared from readily-availablematerials. In general, this is accomplished by reacting suitablealkylated cresols with formaldehyde and isolating the products. Thelatter are substantially colorless crystalline solids.

In most cases, suitable alkylated cresols are commerically available,methods for their preparation being known. Where it is desirable ornecessary to do so, they may be readily prepared by alkylating asuitable cresol'by standard methods. By Way of illustration, forexample. 4-methyl-6-tertiary butyl phenol may be prepared by alkylatingparacresol to introduce a tertiary butyl group in the 2-position. Thismay be done many of several ways. One good method is to pass isobutyleneinto paracresol in the presence of a catalyst such as sulfuric acid orboron fluoride etherate, or a mixture of zinc chloride and phosphoricacid. About five percent of catalyst, based on the expected weight ofalkylate, should be used. Reaction is usually initially exothermic butmay required mild heating to become complete. Similarly, for example,2,4-ditertiary butylphenol may be prepared from phenol and isobutyleneand 2-methyl-4-tertiary butylphenol from orthocresol and isobutylene.

In the following examples, which are given as illustrative only,preparation of several stabilizers typical of the present invention isshown.

EXAMPLE 1 2,2 '-methylene-bis- (4-methyl-6-tertian'bzttylpherzol) 82parts (0.5 mol) of 4-methyl-d-tertiarybutylphenol, parts of concentratedHCl, and 0.2 part of Duponol C are charged to a 500 ml. flask, 100.parts of heptane is added, and the contents of the flask agitated whileWarm- I ing to C. 23 parts of 36% aqueous formaldehyde solution,containing 8.25 parts (0.275 mol) of HCHO, are added dropwise so thatthe temperature does not exceed 60 C. The reaction mixture is digestedat C. for three hours, cooled to 20 C., and the solid product iscollected on a filter. The product is slurried in 500 parts of watercontaining 20 parts of heptane and 0.25% of Duponol, filtered, anddried. The product weighs 73 g.; M. P., 125-128" C., yield 87%. Theaqueous layer is separated from the mother liquor and discarded and 15parts of fresh heptane is added with a fresh charge of 0.5 mol of4-methyl-6-tertiarybutylphenol, 10 parts of concentrated HCl, and 0.2parts of Duponol C. Processing as above, parts of product is obtained;M. P., 123-125 C., yield EXAMPLE 2 2,2'-methylene-bis-(4-methyl-6-iertiary0ctyl phenol 200 parts (0.91 mol) ofZ-tertiaryoctyl-4-methylphenol is mixed with 10 parts of concentratedhydrochloric acid. 42 parts (0.46 mol) of 36% aqueous formaldehydesolution is added thereto and the reaction mixture heated for one hourat 55-60 C. The mixture becomes an unstirrable mass, necesstitating theaddition of 250 parts of heptane as diluent, after which the mixture isstirred for an additional two hours at 55-60" C. The product is thenwashed With water and the hep tane-water mixture is removed byazeotropic distillation. 123 parts of a substantially colorless viscousproduct is obtained. On recrystallization from heptane, a colorlesscrystalline product is obtained which has a melting point of 116- 118 C.

EXAMPLE 3 2,2-mezIzyZeue-bis-( 4-ethyl-6-tertiarybutylphenol) 356 parts(2 mols) of 2-tertiarybutyl-4-ethylphenol and 92 parts of 36% formalinsolution containing 33 parts of CH O were mixed with 2.5 parts ofDuponol C and 350 parts of heptane. 42 parts of concentrated HCl wasadded dropwise and the mixture digested at 50-60 for 3 hours. Theaqueous layer was then separated and the heptane solution of the productwashed with water, caustic and water again until neutral. The solutionwas chilled and 158 parts of crystalline product obtained, which onrecrystallization from heptane gave a product having a melting point of123 C.

Typical of the gel-forming problems involved in the manufacture ofsynthetic rubbers are those found in the making of copolymers of styreneand butadiene-1,3, such as GR-S and similar types. The eflectivness ofthe products of the present invention is clearly demonstrated in thefollowing examples.

EXAMPLE 4 A sample of a commercial synthetic rubber latex, abutadiene-1,3 (76.5 parts) styrene (23.5 parts) copolymer made at 41 F.,was divided into portions. To the first no stabilizer was added; to theseond, 1.25% of phenyl-beta-naphthylamine was added; to the third, 1.25%of 2,2'-methylene-bis-(4-methyl-6-tertiary butylphenol) was added. Theportions were then coagulated by the usual salt-acid method, the solidscollected, and the collected solids washed, then dried at about F. Thegel content was determined. The gel content was then taken after twohours of additional heating at 212 F.

The results are shown in the following table.

It will be seen, therefore, that the stabilizer of the present inventionis superior to the amine-type, not only against gel formation duringdrying, but during heating after drying.

EXAMPLE 5 A latex as in Example 4 is divided into four portions. To thefirst no stabilizer was added; to the second, 1.25 of WingStay-S, one ofthe commercially available more efiective, non-discoloring gelinhibitors was added; to the third and fourth were added 0.4% and 1.25%respectively of 2,2'-methylene-bis-(4-methyl 6 tertiarybutylphenol). Theportions were coagulated by the usual saltacid method and the solidscollected, washed and dried at about 190 F; The gel content wasdetermined. It

. 5 was again taken after 90 minutes at about 250 F. The results areshown in Table II.

TABLE II Percent Gel After rying Amount, Portion Stabilizer Percent atat 190 F. 190 F +90 min.

wt. 250 F.

1 None 2 13.4 2 Wing-St-ay-S (styrene- 1. 25 2 10.1

phtenol condensation produc 3 2,2-methylcne-bis(4-rneth- 0. 4 2

yl-fi-tertlarybutylphenol) 4 (lo 1. 25 0 0 Table II clearly shows thatthe non-discoloring stabilizer according to the present invention is farsuperior to the commercially available non-discoloring stabilizer, evenwhen used in amounts as little as one-third as much.

EXAMPLE 6 The procedure of Example was repeated except the fourthportion was omitted and rather than heating to 250 F. after drying, theportions were stored at room temperature for 8 weeks. Results are shownin Table III.

TABLE III Percent gel Amount, after drying Portion Stabilizer Percent +8weeks at Room Temp.

None 100 Wlng-Stay-S (styrene-phenol 1. 7. 6

condensation product). 3.--- 2,2-Methylene-bis (4-Inethyl-6- 0. 4 2. 0

tertiarybutylphenol).

Table III shows a far greater inhibiting effect by the stabilizer ofthis invention as compared to the commercially available,non-discoloring, stabilizer even when used in substantially smalleramounts.

EXAMPLE 7 Example 6 was repeated except the synthetic rubber was abutadiene-l,3 (76.5 parts) styrene (23.5 parts) copolymer made at 122 F.and the stabilizer according to this invention was2,2'-methylene-bis(4-ethyl-6-tertiary- Again it can be seen that thestabilizer according to this invention is superior to the styrene-phenolcondensation product type, although the 2,2'-methylene-bis(4-ethyl-6-tertiarybutylphenol) of this example is not quite as efiectiveas the 2,2-methylene-bis-(4-methyl-6-tertiarybutylphenol) of thepreceding examples.

While the above examples are concerned'with butadiene-styrene copolymerrubbers of a particular type, the invention is equally applicable toother similar copolymers. It is applicable generally to any situationinvolving a latex prepared by polymerization or copolymerizationfollowed by coagulation. The problem of cross-linkage and gel formationexists, for example, in the manufacture of butadiene-acrylonitrilecopolymers and the like. The process of this invention is equallyefficient in providing effective stabilization in the manufacture ofthat type. It is generally applicable to any polymerization operationwhere the polymerization should be substantially halted and it isthereafter undesirable that further polymerization, cyclization,cross-linkage and gel formation be permitted during drying and/orsubsequent heating and processing.

it is an advantage of the materials of the present invention that-theyare soluble in or readily compatible with the various commercial typesof synthetic rubbers. They are readily incorporated by admixture withthe latex just before coagulation. It is usually most convenient to doso at this point. The materials are non-toxic and produce no untowardskin irritation. They may therefore be handled freely with no specialprecautions.

In general, the use of from about 0.25 to about 2.5 pounds of stabilizerper pounds of polymer or copolymer will be found quite adequate. Ingeneral, about 04-125 percent will be found suitable for most purposes.Where the requirements are not initially known, use of about 1.25 poundsof stabilizer per 100 pounds of finished rubber is a good averagepractice.

We claim: I

l. A synthetic rubber composition stabilized against polymerization,cyclization, cross linkage and the formation therein of an insoluble gelfraction, consisting of an unvulcanized, sulfur vulcanizable syntheticrubber selected from the group consisting of rubber-like polymers andcopolymers of butadiene-1,3 and from about 0.25% to about 2.5% by weightof said rubber of a compound of the formula R: ?H OH I'tz I li R1References Cited in the file of this patent UNITED STATES PATENTS Daviset al. Jan. 16, 1951 Swaney et al. Aug. 28, 1951 OTHER REFERENCES Whiteet al.: Ind. and Eng. Chem.; August 1945, pp. 770-775. (Copy in 260-837)Semon: Chem. and Eng. News; November 10, 1946, pp. 2900-2905. (Copy in260-942.)

1. A SYNTHETIC RUBBER COMPOSITION STABILIZED AGAINST POLYMERIZATION,CYCLIZATION, CROSS LINKAGE AND THE FORMATION THEREIN OF AN INSOLUBLE GELFRACTION, CONSISTING OF AN UNVULCANIZED, SULFUR VALCANIZABLE SYNTHETICRUBBER SELECTED FROM THE GROUP CONSISTING OF RUBBER-LIKE POLYMERS ANDCOPOLYMERS OF BUTADIENE-1,3 AND FROM ABOUT 0.25% TO ABOUT 2.5% BY WEIGHTOF SAID RUBBER OF A COMPOUND OF THE FORMULA